Burrs are often the result of the tree's reaction to a parasitic attack, such as from insects, or can simply be a genetic response to a perceived threat. The mechanism being to produce branch shoots. FAr too many are produce and they never develop, hence the pipping we love to see.
As for turning...treat as normal. Burr can be easier to turn in respect of having no definite grain direction, but crying burrs can be problematic and "pulling" is often seen on burrs turned from green. Most turners don't like the effect but I love it!
